Filters of polyphenylene sulfide (PPS) fibers, fluorine resin fibers or the like are widely used as dust filters for city garbage incinerators or coal boilers. Recently, those incinerators and boilers have become run at high inner temperatures for reducing the amount of dioxin to be generated therein and for increasing the processing efficiency. Therefore, the filters in those are required to have higher long-term heat resistance at high temperatures than conventional ones.
For those filters, proposed were felt of PPS fibers, and felt comprising a substrate of PPS woven fabric having a specifically defined weaving density in which the fabric substrate is sandwiched between webs (see Japanese Patent Application Laid-Open (JP-A) Hei-5-317620). Also proposed were a filter of PPS fabric of which the strength in both the weft direction and the warp direction is specifically defined (see JP-A Hei-3-32711); and a bag filter comprising composite felt of such that webs of PPS fibers are entangled around a PPS spun yarn fabric substrate (see JP-A Hei-9-57026).
However, the proposed filters are all of PPS fibers alone, and PPS has a melting point falling between 270 and 280.degree. C. and the highest temperature for its continuous use is 200.degree. C. in UL Standards. Therefore, their heat resistance and durability are not enough for long-term exposure to high temperatures of 200 to 230.degree. C. or higher.
On the other hand, the particle size of dust to be generated therein is smaller with the increase in the inner temperatures of incinerators and others being run. If the conventional filter material consisting essentially of PPS fibers is modified in order to increase its dust-trapping capacity in such incinerators and others to be run at high temperatures, the air permeability through the filter is lowered, resulting in that the dust as trapped in the filter is difficult to remove and the filter is thereby often clogged.
In order to solve this problem, a filter material has been proposed, which is such that a mixture of organic fibers having a melting point of not lower than 180.degree. C. and glass fibers having a specifically defined diameter is partly bonded (adhered) onto the dust-trapping surface of the filter material (see JP-A Hei-5-245316). Although its dust-trapping ability is good and the dust removal from it is easy, the proposed filter material is still problematic in that the organic fibers and the glass fibers around it separate each other while the filter is continuously used for a long period of time at high temperatures. In such continuous use, the dust-trapping ability of the proposed filter material is lowered and the durability thereof is poor.